非规则封闭声腔在PVDF压电薄膜激励下的响应分析

在噪声主动控制领域,尽管PVDF压电薄膜相对于压电陶瓷而言应变压电常数以及扬氏弹性模量都较小,很少作为作动器使用,但由于它具有良好的柔韧性,能够大面积地覆盖在结构表面,产生大范围的分布应力,能承受较压电陶瓷大得多的电压,因此是一种很有潜力的作动器.为了分析PVDF压电薄膜对于非规则封闭声腔内部声压的控制机理,本文研究弹性边界上有一对压电薄膜作动器激励的非规则封闭声腔响应特性.文中首先以弹性边界的振动方程以及经典波动方程为基础,描述了结构-声之间的耦合作用;然后结合压电薄膜本构方程和弹性边界振动方程,给出了机电耦合的描述.结合这两种描述,本文给出了整个系统在输入交变电场的作用下,如何通过压电薄膜对结构的作用将激励传递到内部声场的理论分析,并且给出了仿真结果.     

空间范数在压电柔性结构振动控制中的应用研究

在空间范数定义的基础上,推导出了结构模态空间范数的计算公式。考虑压电柔性结构振动控制中作动器分布对结构建模与控制性能的影响,利用模态空间范数度量各个模态对结构动力响应的贡献,并对结构进行了模态选择与模型降阶。利用所建立的降阶模型,设计了一个对受外部干扰结构进行振动抑制的动态输出反馈]]>;     

一种弯曲型压电堆作动器的设计及在振动控制中的应用

提出了一种弯曲型压电堆作动器并将其应用于悬臂梁主动振动控制中。新型作动器由一个压电堆和一个 型金属底座和一个预压螺钉组成。该作动器的通过底座对压电堆纵向变形的约束而使底座弯曲变形产生作动弯矩。推导了新型作动器的输出作动弯矩计算公式,并将其应用于悬臂梁振动控制中,采用热弹比拟方法结合状态空间理论建立了带新型压电作动器的悬臂梁振动控制系统状态空间方程。分别应用正位置反馈（Positive position feedback,PPF）和神经网络预测（Neural network predictive,NNP）控制方法设计了主动振动控制系统。闭环仿真结果表明,采用本文提出的新型压电堆作动器控制悬臂梁的一弯模态位移响应,应用PPF控制其幅值可降低57%,应用NNP控制其幅值可降低92%。     

压电层合板高阶计算模型

压电复合(层合)结构可应用于结构振动控制、形状保持、健康监测等,建立压电层合结构精确的机电耦合计算模型成为了研究的焦点.针对表面粘贴或内部嵌入压电片的压电层合板结构,基于高阶位移场和高阶电势模型,根据Hamilton原理建立了机电耦合高阶有限元模型.该模型适用于薄板和中厚板,并且能够捕捉压电层内沿厚度方向呈抛物线型分布的诱导电势.以压电双晶片简支板为例,进行了作动器构型和开环、闭环状态传感器构型的数值分析.结果指出,诱导电势对压电传感器有重要影响,而压电作动器可忽略这种电势.     

书本式压电作动器的特性分析

在考虑粘接层影响的情形下，利用层合梁理论导出了柱形弯曲书本式压电作动器的作动力以及机电耦合系数的数学表达式，并通过数值仿真分析揭示了机电耦合系数与压电层层数、压电层厚度以及粘接层厚度间的关系。这些结果为书本式压电作动器的结构综合优化设计提供了理论基础。     

温度梯度场中梁的形变控制研究

5介绍了一种新型的压电作动器—层叠式压电作动器,并使用这种作动器对温度场中的梁进行了形状控制研究。根据哈密顿原理,得到了粘贴有层叠式压电作动器的梁结构的控制方程,进行了数值仿真,并且用Comsol软件进行了模拟,两者的结果基本一致。对压电作动器的控制电压进行了优化,得到了最优控制电压。由于层叠式压电作动器的控制力与压电片的层数成二次函数关系,当控制电压恒定时,层叠式压电作动器的控制力随着压电器层数的增加而迅速减小。使用层叠式压电作动器可以在比其他作动器更小的电压下取得更好的控制效果。通过与普通压电作动器的比较,可以发现层叠式压电作动器可以有效地降低作动器的施加电压,而且可以显著增强控制效果。这种形状控制方法为应用层叠式压电作动器进行薄壁结构的形状控制提供了理论基础。     

压电作动器对热变形层合板形状修复研究

基于一阶剪切效应Mindlin板理论,建立了在热载下含分布式压电作动器的复合材料层合板有限元分析模型和控制方程,分别研究了该板在内外表面存在温差的情况下的热变形,以及使用压电作动器对热变形区域进行形状修复的问题;在分析中考虑了压电作动器与复合材料层合板间含有胶接层的影响。由典型算例结果讨论,得到如下结论:1)使用压电作动器可以有效地对复合材料层合板的表面热变形形状进行修复;2)压电作动器的分布位置对修复效果影响很大;3)在电压达到一定数值后,继续增加电压值对修复效果贡献很小。     

结构的主,被动振动控制及相互影响分析

同时采用被动和主动控制方法来抑制结构的振动，可以利用两者的优势以达到更好的控制效果。以粘弹性阻尼器和压电晶体作动器为被动和主动控制元件，建立了主结构与控制元件一体化的复合结构运动控制方程，以平面桁架结构为例进行了数值分析，研究了控制器设计和系统参数选择、主动振动控制的效果以及被动振动控制与主动振动控制的相互影响等问题     

一种混合遗传算法在压电智能结构振动控制全局优化设计中的应用

本文针对采用压电作动器／传感器的智能结构,了振动控制中作动器／传动器的配置以及反馈增益的全局优化设计问题。     

基于模态应变能分布的压电致动器/传感器位置优化遗传算法

本文由线弹性压电结构有限元动力方程,推导了压电智能结构的振动控制方程。建立了准确模拟层合压电结构动力行为的有限元模型。基于主结构模态应变能分布提出了一种新的优化目标函数,将压电致动器/传感器位置编号作为优化变量,建立了离散变量表示的智能结构优化问题,并通过二进制编码的遗传算法（GA）求解了该最优问题。以四边固支复合层合压电智能板为数值算例,采用比例反馈控制, 研究了最优位置配置致动器/传感器智能结构目标模态的控制效果。数值结果表明基于模态应变能分布的遗传算法所得优化解具有较好的振动控制效果。     

Complex Impedance of Fast Optical Transition Edge Sensors up to 30 MHz

Optical transition edge sensors (TESs) are characterized by a very fast response, of the order of \(\upmu \)s, which is \(10^3\) times faster than TESs for X-ray and gamma-ray. To extract important parameters associated with the optical TES, complex impedances at high frequencies (> 1 MHz) need to be measured, where the parasitic impedance in the circuit and reflections of electrical signals due to discontinuities in the characteristic impedance of the readout circuits become significant. This prevents the measurements of the current sensitivity \(\beta \), which can be extracted from the complex impedance. In usual setups, it is hard to build a circuit model taking into account the parasitic impedances and reflections. In this study, we present an alternative method to estimate a transfer function without investigating the details of the entire circuit. Based on this method, the complex impedance up to 30 MHz was measured. The parameters were extracted from the impedance and were compared with other measurements. Using these parameters, we calculated the theoretical limit on an energy resolution and compared it with the measured energy resolution. In this paper, the reasons for the deviation of the measured value from theoretically predicted values will be discussed.     

The energy reflection coefficient of electrorheological fluid with continuously changing acoustic impedance

The electrorheological fluid (ERF), among the complex fluids, is a kind of smart or intelligent material. At the presence of an external electric field, the dispersed particles in ERF suspensions can form orderly microstructures to a certain extent, resulting in the drastic changes of many physical properties of the materials, such as acoustic impedance. This paper analyzes acoustic attenuation and velocity of acoustic wave in ERF, as a theoretical model the Biot theory has been applied. Then impedance-matching properties of ERF, whose specific acoustic impedance varies smoothly across the ERF, from the effective value of the transducer specific acoustic impedance to the value of the output medium specific acoustic impedance, is analyzed with the transmission matrix method and get the reflection coefficient of ERF. At last, calculate the reflection coefficient of ERF with three impedance profiles. The results show that This impedance matching technology (IMT) is the most effective for active noise control.     

A Novel Technique of Flow Measurement for a Conducting Liquid

The flow rate of a conducting liquid is generally measured by an electromagnetic flowmeter, which requires a magnetization coil and a set of sensing electrodes. However, the size and cost of the flow head of this flowmeter generally become very high due to the large size of the magnetizing coil and core material. In this paper, a novel low-cost bridge-type technique of flow measurement of a conducting liquid has been described. In this technique, the lumped-parameter impedances among four electrodes placed at a radial distance apart in a flow-sensing tube form a Wheatstone bridge network. Each of these impedances is a combination of electrode polarization impedances and other impedances. Since the electrode polarization impedances are functions of the flow rate of the conducting liquid and some other parameters, the polarization impedances between two electrodes placed in two locations along the flow path of the conducting liquid may be taken as a measure of the flow rate of the liquid, with the other parameters remaining constant. The nonlinearity of these impedance variations with the flow rate is minimized by the proposed bridge-circuit near-balanced condition. The bridge is excited by a stabilized sinusoidal oscillator and balanced at the no-flow condition of the liquid. The output of the bridge network is converted into a 1- to 5-V dc signal in a signal conditioner circuit. The design aspects, theoretical analysis, and calibration data are presented in this paper. The experimental characteristic of the flowmeter is found to follow the theoretical equation and has good repeatability over the entire operating zone.      

Derivation of COM equations using the surface impedance method

The surface impedance method is used for the consistent derivation of coupling of modes equations which describe the interaction of SAW with a periodical system of electrodes of finite thickness. The exact analytic solution of the electrostatic problem in the presence of an arbitrary external electric field for a plane system of electrodes is applied to the calculation of the charge and electric field distributions. Mechanical perturbations are taken into account to first order of the thickness of the electrodes. As a result the scalar self-consistent equation for the electric potential of acoustic waves in the gratings is obtained. For the periodic structure this equation is reduced to the form of COM equations for slowly varying amplitudes. Analytical expressions for all coefficients of the COM equations connecting them with geometrical and material parameters are found. The NSPUDT effect can be considered. Dissipation and energy storage terms can be introduced empirically. The solution of the COM equations is represented in the form of a P matrix with elements written in a convenient form. A simple formula for calculating the location of maximum transducer frequency response is proposed, The balance of energy is considered. Some new relations among the elements of P matrix are found     

Fault Detection of a Rotating Shaft by Using the Electromechanical Impedance Method and a Temperature Compensation Approach

Condition-based maintenance systems that use sensor networks for damage detection in rotating machinery have been evolving constantly. Such strategies aim at detecting the presence and severity of damage on a statistical basis. The aim of this contribution relies on the correct detection of incipient faults in rotating shafts by using a real-time impedance-based structural health monitoring method, with a low-cost portable device. This technique monitors changes in the electric impedance of piezoelectric transducers, acting simultaneously as actuators and sensors, which are bonded to the host structure. With the use of damage metrics, these changes can be quantified so that the presence and severity of damage are detected. This is possible since the electrical impedance of the sensor is directly related to the mechanical impedance of the structure. However, the frequency response functions resulting from this method are susceptible to environmental and operational conditions that should be accounted for to avoid false diagnostics. Consequently, a temperature compensation technique is proposed based on a hybrid optimization method associated with different damage metrics. Additionally, a statistical model is used for threshold determination based on the statistical process control method. Experimental results show that an incipient fatigue crack associated with bearing wear was successfully detected with a probability of detection above 95% confidence for the majority of sensors used.     

The dynamic behavior of a surface-bonded piezoelectric actuator with a bonding layer

The performance of smart structures depends on the dynamic electromechanical behavior of piezoelectric actuators and the bonding condition along the interface, which connects the actuators and the host structures. This paper provides a theoretical study of the influence of material parameters of the bonding layer on the coupled electromechanical characteristics of piezoelectric actuators, which are subjected to high frequency electric loads. A one-dimensional actuator model with a bonding layer, which undergoes a shear deformation, is proposed. Analytical solutions based on the integral equation method are provided. Detailed numerical simulation is conducted to evaluate the effect of the bonding layer under different loading frequencies. The results indicate that the properties of the bonding layer, the loading frequency, the material combination and the geometry of the actuator have a significant effect on the load transfer between the actuator and the host medium.     

Rectangular microstrip antennas

Abstract

The theory of rectangular microstrip antennas based on the line resonator model and the cavity model are summarized. The theoretical and experimental values of input impedance and radiation patterns are compared, and the discrepency between these two values are explained. A parallelogram microstrip antenna is constructed, and the input impedances are compared to those of the rectangular microstrip antenna.     

Eddy current inversion for layered conductors

By making multifrequency eddy current measurements on a layered conductor, it is possible to acquire information on the depth dependence of the conductivity. We consider an inversion problem in which coil impedance data are used to determine either the layer thicknesses or layer conductivities. The algorithm is based on a well known forward model which gives the impedance of an air cored coil above a stratified conductor from a closed form expression. In the forward calculation, estimates of the unknown material parameters are used to get tentative predictions of the measurements. Differences between these predictions and measured impedances are expressed in terms of a global error that is minimized iteratively with the aid of a descent algorithm by varying the parameters of the structure. Examples of minimization searches for layer parameters are given.This article is dedicated to Professor Bertram A. Auld on the occasion of his 70th birthday and his retirement from Stanford University.     

Evaluation of the material parameters of piezoelectric materials byvarious methods

The elastic, dielectric and piezoelectric constants of four piezoelectric materials, including polyvinylidene fluoride, vinylidene fluoride-trifluoroethylene copolymer, PZT/epoxy 1-3 composite, and lead metaniobate ceramic, have been evaluated from the impedance data using five different methods. A method described in ANSI/IEEE Std. 176-1987, though based on formulae derived for loss-less materials, is found to be applicable to materials with moderate loss. However, for high-loss materials such as polyvinylidene fluoride, the electromechanical coupling constant (κ_t) obtained by the method of Std. 176 is substantially higher than the actual value. Calculations based on a piezoelectric resonance analysis program (PRAP) combine the best features of two earlier methods. In addition to the impedance at the parallel resonance frequency, impedances at two other frequencies are required for calculation. The PRAP method gives quite accurate material parameters regardless of the magnitude of the loss, but the parameters (including κ_t) vary by as much as 15% depending on the choice of data. In the nonlinear regression method described in the present work, all the impedance data points around the resonance are least-squares fitted to the theoretical expression for the impedance. Besides the advantage of requiring no arbitrary choice of data, the nonlinear regression method can readily take account of the frequency dependence of the dielectric constant     

Identifying the Magnetic Part of the Equivalent Circuit of n-Winding Transformers

Representation of multiwinding transformers by equivalent circuits has been recently improved, and it is for the identification of components of these circuits. In this paper, the focus is on magnetic coupling with its related losses. A general method, based on external impedance measurements, is followed to determine inductances, coupling ratios, and resistances included in these equivalent circuits. Justification for impedance measurements, choice of measured impedances, and precautions regarding short-circuit compensation are discussed. For illustration, two components are tested, and their equivalent circuits are established